Satoshi planned for this 51% mining attack problem, and so far we're ok.
His premise was that if a miner ever grew so large as to be capable of performing a 51% attack, that they would have so much wealth invested in the Bitcoin network that they would never dare try anything.
The Bitcoin network is GHash's golden egg laying goose.
The Bitcoin network is entirely safe in GHash's hands imho.
The only real worry comes in when the 51% miner is a bad actor.
>at 35% hashpower and 3 confirmations, this means that GHash can currently steal altcoins from an altcoin exchange with 15.6% success probability – once in every six tries.
Yes, but come on. They never would. Would Bill Gates jump into an active volcano to pick up a penny? Even if he had a 15.6% of picking up that penny without dying a fiery death?
>So what if, for the sake of example, GHash gets over 51% again and starts launching 51% attacks (or, perhaps, even starts launching attacks against altcoin exchanges at 40%)? What happens then?
I'll bite. The ~75% of GHash's mining power that is being pointed at GHash by regular Bitcoin ASIC miners who are enjoying their 0% fees and merged mining drops off overnight. Their domain names are DDOSd at a record breaking level for weeks on end, and every other pool adopts a modified bitcoind that hard-forks (erases) any damange GHash might have caused.
GHash is left with a few thousand nearly worthless Bitcoin (because the USD/BTC value would tank on their 51% attack news) and they are also left without the possibility to basically print themselves free money until the end of time.